Many days ago I asked you to hook any passive headphones/small PC speakers to your soundcard jacks and listen if the noise is still there. That test will show if the noise is present at the soundcard input (you will hear it in the headphones) or if it is some kind of ground loop problem (you will not hear the noise in the headphones since they have only single pair of connecting wires and no ground loop can occur).

Problems need to be diagnosed first, before taking corrective actions.

Ah, I think that a first step is to open the computer's audio mixer and run everything except main and wav, all the way to the bottom, and/or mute everything except main and wav.

To set the main and wav, I think we'd try to identify the make/break point between loss and overload, as a sort of "ballpark" place to start. I'd run them down from max until a tonal/pressure change/loss is heard and set them somewhere near that or just fractionally higher than the loss point. It may be about 60%. You can also try RMAA software, which is free and will guide you into some functional settings without overload. Try adjusting your playback software up to 24bit output instead of 16bit output.

Set the Input mixer (often hidden--in menu properties, etc. . . ) to anything other than "waveout" or "what u hear" since internal looping, is sometimes the factory default, but can make the worst performance possible. RMAA will help reveal what's going on. My X-Fi's factory default setting was the junk (digital noise gain?) setting, and it was fixed by setting the Input mixer to line in and setting the Output to mute the line in's playback.

If that doesn't do it, you could choose an amplifier less sensitive to computer noise. A non-inverting amplifier with shunt comp, like Circlophone, is less likely to amp computer noises. However, an inverting amplifier with miller comp, is most likely to amp computer noises. I wish there was a "buffered shunt-comp preamp" available!

The applicability of the above comments, depends, in part, on the output of the sound card or sound chip because if the computer sound has an onboard preamp that has already increased the computer noise, then neither swapping the power amp nor adjusting the sliders would be sufficient. So, the third step, if needed, is actually pretty big--no analog audio parts running on the computer power supply.

P.S.
For integrated motherboard sound chip, sometimes the front panel cables are not fun for hi-fi. In this case, you can look up how to disconnect that cable and bypass with two jumpers so that the sound goes directly to the rear jacks without running across the computer twice.

When the noise is power circuit related, you can. . .
Output only digital to an external sound module that doesn't take any power from the computer (a rather high end deal built to isolate against computer noise).
Or,
Optical output to an external sound module
Or,
Change the computer's power supply
Or,
Change the computer's motherboard.

- Changed to a different PCIex Slot (no change)
- Changed the Juli@ card from RCA to jack connections (no change)
- Plugged the card directly into my amp instead of my mixer (no change)
- Tried another wall socket (no change)
- Moved the system to the groundfloor and tried a socket there (no change)

Then it hit me! My brothers has the same mainboard (Asus P8B75-v) and installed the Juli@ card in his system. Same issue!

So it was safe to say that the mainboard was the issue. I've ordered a new mainboard (non Asus and another chipset) and the problem i had is now gone. I've mailed the issue and solution to the support department of ESI.

Another guy on this forum had the same problem on a different soundcard, I told him to power the soundcard from an external clean supply and this worked well... of course this needs either a soundcard with power connector (usually floppy connector), or some hacking of the soundcard itself...